1. Field of the Invention
The present invention relates to a fuel cell stack comprising a fuel cell unit including a solid polymer electrolyte interposed between an anode electrode and a cathode electrode, and first and second separators for holding the fuel cell unit therebetween.
2. Description of the Related Art
For example, a fuel cell of the solid polymer electrolyte type includes an anode electrode and a cathode electrode disposed opposingly on both sides of an electrolyte membrane composed of a polymer electrolyte (cation exchange membrane) to construct a fuel cell structure (hereinafter referred to as "fuel cell unit") which is held between separators. In general, a predetermined number of the fuel cell units are stacked to provide a fuel cell stack which is practically used.
Such a fuel cell is operated as follows. That is, a fuel gas, for example, hydrogen, which is supplied to the anode electrode, is converted into hydrogen ion on electrode catalysts. The hydrogen ion is moved toward the cathode electrode via the polymer electrolyte which is appropriately humidified. Electrons are generated during this process, which are extracted by an external circuit to be utilized as direct current electric energy. An oxygen-containing gas, for example, oxygen gas or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electrons, and the oxygen are reacted with each other on the cathode electrode to produce water.
In order to supply the fuel gas and the oxygen-containing gas to the anode electrode and the cathode electrode respectively, conductive porous layers such as porous carbon paper sheets are disposed on electrode catalyst layers (electrode surfaces), and they are interposed between the separators. One or a plurality of gas flow passages, each of which is designed to have a uniform widthwise dimension, are disposed on mutually opposing surfaces of the respective separators.
However, in the case of the fuel cell constructed as described above, the number of reactive molecules per unit area existing in the vicinity of an outlet of the gas flow passage decreases as compared with the number of reactive molecules per unit area existing at an inlet of the gas flow passage, because the fuel gas and the oxygen-containing gas supplied to the gas flow passage are consumed in the electrode surface. Accordingly, a problem occurs that the reaction in the electrode surface becomes non-uniform, and the cell performance becomes unstable.
An amount of condensed water and an amount of water produced by the reaction sometimes exist in the gas flow passage in a state of liquid (water). It is feared that if the water is accumulated in the porous electrode layer, then the performance to diffuse the fuel gas and the oxygen-containing gas to the catalyst electrode layer is lowered, and the cell performance is markedly deteriorated.
In this context, for example, a fuel cell is known, as disclosed in Japanese Laid-Open Patent Publication No. 6-267564. The fuel cell includes a fuel-delivering plate having a fuel flow passage for supplying fuel to an anode electrode, and an oxygen-containing gas-delivering plate having an oxygen-containing gas flow passage for supplying an oxygen-containing gas to a cathode electrode, in which at least any one of the depth or the width of the oxygen-containing gas flow passage of the oxygen-containing gas-delivering plate gradually decreases from an upstream flow passage region to a downstream flow passage region.
However, in order to sufficiently supply the fuel gas and the oxygen-containing gas to the electrode surfaces respectively, the gas flow passage is provided in a serpentine manner or in an encircling manner in a surface of the separator. Therefore, the gas flow passage is considerably lengthy in the separator surface. In the case of the conventional fuel cell as described above, the depth of the oxygen-containing gas flow passage is large in the upstream flow passage region, and hence the separator itself is considerably thick-walled. Accordingly, a problem is pointed out that it is not easy for the entire fuel cell to achieve a compact size. Further, a problem arises in that the machining operation for manufacturing the gas flow passage to have the depth which gradually decreases from the upstream to the downstream is extremely complicated.